The invention discloses a supplemental log parsing method relating to before-images of a database and the method comprises the following steps: (1) obtaining version information of the database, parsing the KDO data of a forth module of the change of before-images, and extracting operation codes; (2) adopting different parsing patterns according to different operation codes, and obtaining the code of the supplemental log data; (3) obtaining the data of the primary key columns inside the supplemental log according to the structure of the supplemental log; (4) adding the obtained data of the primary key columns to the data of changed columns in proper order. By the supplemental log parsing method, the line number of the changed data is precisely positioned according to the primary key, and simultaneously the performance of the data itself is not affected.